[ISSUE] Deleting mseq selections

[jeremybobbin <jeremybobbin@users.noreply.github.com>]

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New issue by jeremybobbin on mblaze repository

https://github.com/leahneukirchen/mblaze/issues/182

Description:
Say I delete the selected thread from my inbox:
```sh
mseq -r .= | grep "Inbox" | mflag -S | xargs rm
```

After doing this, the thread will remain in `$MAILSEQ`, so I need to either "re-index" my mail and their threads:

```sh
full_thread() {
	tmp="$(mktemp)"
	mdirs "$@" | xargs mlist | mthread | tee "$threads" | MAILSEQ="$tmp" mseq -S >/dev/null
	grep -nFf "$patterns" "$threads" | sed 's/:.*/=/' | MAILSEQ="$tmp" xargs -n1 mseq | awk '!x[$0]++'
}

mlist -s ~/.mail/*/Inbox | full_thread ~/.mail/ | mseq -S && mseq -C $
```

Or, I'd need to remove the selection from `$MAILSEQ`:

```sh
thread=$(mktemp)
old=$(mktemp)
mseq .= > "$thread" 
cp "${MBLAZE:-$HOME/.mblaze}/seq" "$old"
grep -vFf "$thread" "$old" | mseq -S  
```

I was curious as to whether you had any ideas on how do any of these things more effectively,
as well as your thoughts on an "invert matches" option like `grep`'s `-v`.

Thanks